1. Field of the Invention
The present invention relates to a resistance-change memory using a resistance-change memory element that discriminates between “0” and “1” data by the resistance change of the element and, more particularly, to a circuit used to read out a reference signal generated by a reference cell.
2. Description of the Related Art
Examples of a resistance-change element that stores data by using the resistance change of the element are an MRAM (Magnetic Random Access Memory), PRAM (Phase-change Random Access Memory), and ReRAM (Resistance Random Access Memory) (e.g., U.S. Pat. No. 7,105,870 and IEEE Journal of Solid-Static Circuits, Vol. 38, No. 5, May 2003, pp. 769-773). A resistance-change memory will be explained below by taking the MRAM as an example.
The MRAM is a memory device formed by an MTJ (Magnetic Tunnel Junction) element using the magneto resistive effect by which the resistance value changes in accordance with the magnetization direction. In particular, a large resistance change can be obtained by an MTJ element using the TMR (Tunneling Magneto Resistive) effect.
The conventional MRAM uses the induced magnetic field writing method that reverses the magnetization direction in a free layer by using an induced magnetic field generated by supplying an electric current through an interconnection running near the MTJ element. However, this method has the problem that a write current increases as micropatterning advances. This problem is serious when implementing a large-capacity memory.
As a new write method of solving this problem, the spin transfer torque writing method has been proposed (e.g., U.S. Pat. No. 5,695,864). The spin transfer torque writing method reverses the magnetization in a free layer by directly supplying an electric current to the MTJ element. The direction of the electric current determines the relative magnetization direction in the free layer with respect to a pinned layer. In this spin transfer torque writing method, the electric current for reversing the magnetization in the free layer decreases in proportion to the size of a cell, so the write current reduces as micropatterning advances. Accordingly, the spin transfer torque writing method is suitable for a large-capacity memory.
The MTJ element has a resistance value Rmin or Rmax (Rmax>Rmin) in accordance with whether the magnetization directions in the free layer and pinned layer are parallel or antiparallel. In the read operation of the MRAM, a read current is supplied to a read object cell, and the change in electric current or voltage corresponding to the resistance value of the element is compared with a reference signal and read out. The reference signal can be generated from an external circuit, or from reference cells in which “0” and “1” data are prewritten. Unfortunately, the method of generating the reference signal from an external circuit consumes an extra space and extra power. Therefore, it is preferable to use a portion of a memory cell array as the reference cell.
When reading out data from the MRAM by using the reference signal, the read margin can be maximized by using middle resistance Rmid=(Rmax+Rmin)/2. U.S. Pat. No. 6,392,923 achieves Rmid by combining a plurality of reference cells. More specifically, the middle resistance Rmid is obtained by connecting series circuits of Rmax and Rmin in parallel. Also, an MRAM is implemented by using the reference signal of this type (e.g., IEEE Journal of Solid-Static Circuits, Vol. 38, No. 5, May 2003, pp. 769-773).
Unfortunately, the read circuit of the conventional spin transfer torque writing type MRAM as described above has the following problems.
First, when the conventional reference signal generation method is applied to, e.g., the spin transfer torque writing MRAM, a reference cell having the resistance Rmax and that having the resistance Rmin must be connected in series. For this purpose, a special interconnection must be formed in only a reference cell portion of the memory cell array by using a dedicated reference cell formation process. In addition, series-connected reference cells are connected in parallel in order to achieve one reference signal. This requires two data rows and two data columns. Accordingly, the occupied area of the cell array increases.
Also, unlike in the induced magnetic field writing method described in U.S. Pat. No. 6,392,923, the spin transfer torque writing method has no means for writing different data in the two series-connected reference cells. In the stage of forming the reference cells, therefore, data corresponding to Rmax and Rmin must be prewritten in the reference cells. This decreases the degree of freedom of reference cell data setting.
Furthermore, in the spin transfer torque writing type MRAM, an electric current is supplied to the MTJ element in a read operation in the same manner as in a write operation. This increases the possibility of so-called read disturb by which a write error occurs during a read operation. Especially in the method of generating the reference potential from the reference cells in a read operation, the probability of the read disturb is highest because the reference cells are frequently accessed. That is, a read current is supplied to the reference cells in the same direction regardless of whether the data of each reference cell is “0” or “1”. Therefore, the write current and read current flow through one reference cell in the same direction, and flow through the other reference cell in opposite directions. Accordingly, the read disturb readily occurs in the reference cell in which the latter data is written.